1. Field of the Invention
The invention relates generally to communication systems and, more particularly, to testing the dynamic range of a fiber-optic communication system.
2. Discussion of Related Art
In today""s information age, there is an increasing need for high speed communications for an ever-increasing number of communications consumers. To that end, communications networks and technologies are evolving to meet current and future demands. Specifically, new networks are being deployed which reach a larger number of end users, and protocols are being developed to utilize the added bandwidth of these networks efficiently.
In order to meet the communications demands of its customers, many cable companies are planning to support new information services such as voice telephony and high-speed data services. Unlike the traditional broadcast television services provided by the cable companies, these new information services typically require two-way communications capabilities. Therefore, many of the cable companies are upgrading their existing cable plants with two-way hybrid fiber-optic/coaxial cable (HFC) networks that will support the full-duplex, high-bandwidth applications of today and the future.
An exemplary HFC network for supporting both traditional broadcast video and new high-speed data services is shown in FIG. 1. The HFC network includes headend equipment 101 situated at the headend of the cable plant, signal processing equipment 1041-104n situated at the customer premises (collectively referred to as customer premise equipment 104), and an HFC network 102 comprising the communication network between the headend equipment and the customer premise equipment. For convenience, the signal path from the headend to the customer premises is referred to as the xe2x80x9cforward path,xe2x80x9d while the signal path from the customer premises to the headend unit is referred to as the xe2x80x9creturn path.xe2x80x9d
The forward path is divided into a number of channels, where each channel is allocated a specific frequency band. In a typical HFC network, the forward path channels are allocated 6 MHz of bandwidth within the frequency range 50 MHz to 1 GHz. Some of the forward path channels are used to carry analog video carriers while other forward path channels are used to carry the digitally modulated carriers for the forward path data services. Each forward path data channel is supported by a headend transmitter (e.g., headend transmitter 106n) which formats and modulates user information for transmission to the customer premises over the HFC network. For convenience, the data channels carried in the forward path are referred to as xe2x80x9cdownstream channels.xe2x80x9d
Likewise, the return path is divided into a number of channels, where each channel is allocated a specific frequency band. In a typical HFC network, the return path channels are allocated between 200 KHz and 6.4 MHz of bandwidth within the frequency range 5 MHz to 42 MHz. The return path channels are used to carry the digitally modulated carriers for the return path data services (the broadcast video service typically does not require any return path services). Each return path data channel is supported by a headend receiver (e.g., headend receiver 108n) which demodulates the signal received from the customer premises. For convenience, the data channels carried in the return path are referred to as xe2x80x9cupstream channels.xe2x80x9d
In the forward path, the digitally modulated carriers from the headend transmitters 106 are multiplexed together with the analog video carriers into a composite signal by multiplexer 110. The composite signal is carried by the HFC network 102 to the customer premises equipment 104. The HFC network 102 includes a forward path laser 112, which receives the composite signal from the headend equipment at a predetermined signal level and transmits the composite signal over the fiber-optic portion of the network to a forward path optical receiver 114. The forward path optical receiver 114 converts the optical signals from the forward path laser 112 into electrical signals and passes the electrical signals on to the coaxial cable portion of the network at two-way repeater 116. The coaxial cable portion of the network consists of a number of coaxial cable segments linked together, with two-way repeaters (such as two-way repeater 118) spaced at predetermined intervals along the coaxial cable segments to amplify the signals. Each customer premise connects into the coaxial cable network by way of a tap such as tap 120.
In the return path, the digitally modulated carriers generated at the customer premise equipment 104 are transmitted over the coaxial cable network by the series of two-way repeaters, such as two-way repeater 118. At the boundary between the coaxial cable portion of the HFC network and the fiber-optic portion of the HFC network, two-way repeater 116 passes the electrical signals on to return path laser 122 at a predetermined signal level. The return path laser 122 transmits the signal over the fiber-optic portion of the network to a return path optical receiver 124, which converts the optical signals from the return path laser 122 into electrical signals and passes the electrical signals on to the headend equipment 101. The headend equipment 101 includes a splitter, which feeds the return path signals to the headend receivers 108.
One factor affecting performance of the HFC system is the dynamic range of the lasers. Each laser has a power saturation point (PSAT) below which the laser is predominantly linear and above which the laser is severely non-linear. In order to effectively transmit user information, the transmit signal level applied to the laser must be within the linear range of the laser. Thus, the transmit signal level at which the laser reaches its PSAT represents a maximum transmit signal level that can be applied to the laser.
Another factor affecting performance of the HFC system is the inherent noise floor of the optical receiver. Each optical receiver has an inherent noise floor which represents the lowest receive signal level that the optical receiver can process reliably. The receive signal level present at the optical receiver is a function of the transmit signal level applied to the laser, the output power level of the laser, and the optical path loss. In order to effectively receive user information, the receive signal level must be above the optical receiver""s inherent noise floor. Thus, the transmit signal level which produces a receive signal level equal to the optical receiver""s inherent noise floor represents the minimum transmit signal level that can be applied to the laser.
An additional factor affecting only the return path in the HFC system is an accumulation of noise at the headend receiver caused by combining multiple return paths. It is common practice in current HFC systems to combine multiple return paths as shown in FIG. 2 in order to reduce the number of headend receivers needed to support the return path data service. The result of combining multiple return paths is that the summation of noise (a significant portion of which is caused by the optical receiver noise) from all of the return paths is present at the headend receiver. In order to effectively transport user information over the return path, the signal level for the user information must be above the cumulative noise. Thus, the transmit signal level which produces a receive signal level at the headend receiver equal to the cumulative noise represents the minimum transmit signal level that can be applied to the laser on the return path.
As a result, there is a window within which the transmit signal level applied to the laser must remain. This transmit signal level window is bounded at the high end by the maximum transmit signal level and at the low end by the minimum transmit signal level. Because the transmit signal level applied to the laser is itself a range of signal levels, it is imperative that the range of transmit signal levels applied to the laser be situated well within the transmit signal level window so that the strongest signal applied to the laser will not exceed the maximum transmit signal level and the weakest signal applied to the laser will not fall below the minimum transmit signal level.
In order to select an appropriate transmit signal level for the laser, the dynamic range of the laser (i.e., the maximum and minimum transmit signal levels) must be ascertained. In existing HFC systems, there is no reliable way for determining the dynamic range of the laser. Therefore, a need remains for a method and apparatus for evaluating dynamic range in an HFC system.